Volume 2 Number 12 Dennis R. Dinger 1 October 2004
An Update
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The topic of the main article in this issue is the third in a series discussing the use of one type of machine to do the work of another.
Using a Holding Tank as a Blunger
The title of this article may be somewhat misleading -- I don't know anyone who actually thinks they can blunge their slip in a holding tank -- but I know lots of engineers who use holding tanks to perform the work best performed by blungers.
Blungers
Whenever I use this word, lots of non-ceramists and new ceramists scratch their heads and ask themselves, "What's he talking about?" So I will define blunge and blunger right at the start. To blunge, a verb, is to use a relatively high shear impellor blade in a tank to mix dry powders into water (or into another carrier fluid) to achieve a well-mixed suspension. A blunger, a noun, is the device that performs the blunging. A standard milkshake mixer, when used to make a diet milkshake by mixing a dry, diet powder into water or milk, performs as a blunger. Production blungers can be relatively large -- holding thousands of gallons of suspension. All ceramic companies that make and/or use slips in their production processes generally have one or more production blungers on site as well as a variety of smaller lab and pilot scale units.
Production blungers are usually cylindrical, but some are hexagonal or octagonal depending on the materials from which they are constructed. Some are stainless steel while others are concrete. Many concrete blungers are lined with heavy refractory brick to prevent abrasion at the walls. Some concrete HID (high intensity dispersion) blungers are lined with heavy refractory brick, with some bricks turned 90o so they stick out into the tank. When the whole side walls of these tanks are covered with such bricks that stick out into the tank, the suspension is prevented from sliding at the tank wall. In smooth cylindrical tanks, as suspension, powder, and water are sheared by the impellor, the tendency is for the suspension to spin within the tank and slide against the walls. When sliding occurs in smooth cylindrical tanks, the full benefits of the high speed impellor are not realized. In refractory linings containing bricks sticking out into the tank in every few square feet of wall surface area, these bricks 'grip' the slip at the wall and help to prevent the suspension from sliding. This actually creates a 'perfectly' rough wall surface which forces the suspension to be sheared between the impellor and the wall. Baffles inserted near the wall in smooth, cylindrical tanks perform similarly.
Holding Tanks
By comparison with blungers, holding tanks take a variety of shapes and sizes. They are usually quite large and they use relatively low speed impellors. Holding tanks frequently can handle thousands of gallons of suspension. A single holding tank usually holds the combined volume of suspension from several production blunger batches. The purpose of a holding tank is to hold suspension (as the name implies) in preparation for use in the production process.
Impellors in holding tanks should provide gentle agitation and specifically, they should produce vertical recirculation within the tank. The purpose of the agitator is to prevent settling of large particles. Actually, large particles cannot be prevented from settling (unless we can turn off gravity). So the purpose of impellors in holding tanks is to provide recirculation flow -- from bottom to top -- which will carry large particles (that have settled to the depth at which the impellors are located) back to the upper levels in the tank. Typical flow in a holding tank should be downward near the impellor shaft and upward near the walls.
This gentle recirculation from bottom to top to bottom to top, etc., also allows the suspension to be exposed to air at the suspension surface. This can help to remove air bubbles (by bringing them to and leaving them at the surface) and to equilibrate the suspension by exposing all ingredients to air. Some organics pick up oxygen and achieve equilibrium as they are exposed to surface air.
Impellors
The impellors used in blungers should produce intense shear. Some impellors look like the propellors on high speed motor boats. Some have relatively flat carbide surfaces that bat suspended particles around. Some impellors look like standard wood saw blades that have had their blade tips bent down at 90o angles so they resemble the shapes of hole saws. Some impellors operate within concentric stators so they produce enormous amounts of shear as suspended particles are sheared in the relatively narrow gaps between rotors and stators.
The impellors used in holding tanks are far more benign. Such impellors should be designed to cause vertical circulation as mentioned above. Such impellors are usually located near the bottoms of the holding tanks (sometimes with more impellor blades located at shallower depths above.) Figure 1 shows the type of impellor we have recommended for holding tanks.
Figure 1. A Recommended Holding Tank Impellor
Notice that the two impellor blades are located at 90o to each other at two depths in the tank. The blades at the ends of the two horizontal arms are pitched slightly to produce some upward flow. The downward flow near the center of the tank at the shaft will occur naturally due to the upward flow near the walls.
Figure 2. A Water Treatment Impellor
We frequently have found impellors of the type shown in Figure 2 in ceramic suspension holding tanks. These impellors are designed for water treatment tanks in which you want the particles to flocculate and settle out. Notice that this type of impellor produces no vertical flow. It only spins the fluid in the tank which allows sedimentation to occur throughout the tank. In water treatment facilities, this is good. In ceramic suspensions, this is bad. In holding tanks for ceramic suspensions, the impellors must produce some upward flow or all particles with equivalent spherical diameters greater than about one micrometer will settle. Without vertical recirculation, the specific gravity of the suspension near the bottom of the tank can be quite high, and the specific gravity of the suspension near the surface can be correspondingly low. When the goal is to produce consistent product from day to day, this condition (varying specific gravity with depth in tank) is totally undesirable.
Adjusting Additives in Holding Tanks
This section of the article covers the main reason for the article. If we all agree that blunging and mixing should be performed in the best possible devices for these purposes, then the adjustment of slips and the mixing of additives into production suspensions should always be performed in the main blunger tank (which was designed for mixing.) This, however, is frequently impractical because the suspension to be tuned is in a storage tank and the main blunger is being used to produce another batch. Also, the time involved to pump suspension from storage tank to blunger, perform the tuning, and return the suspension to the storage tank can be prohibitive. And finally, the simple process of repeatedly pumping suspension from one tank to another is prohibitive in and of itself.
The introduction of additives into a holding tank must be done carefully and sufficient time must be allowed for mixing to approach completion -- in a holding tank, this could take days to accomplish. When a continuous high intensity dispersion (CHID) loop or a stirred ball mill (SBM) loop is already part of the preparation process, chemicals can be injected prior to the suspension entering the CHID or SBM. Such continuous SBM or CHID loops are not usually installed with storage tanks. If a pumping loop can be used in conjunction with a storage tank, chemicals can be injected into the suspension prior to the pump so mixing will occur within the pump and within the pipe loop that returns suspension to the holding tank. Alternatively, a small sample (e.g., 5 gallons) of suspension can be removed from the tank, and chemicals can be added as the sample is mixed on a lab-scale mixer. The sample of suspension/additive mixture can then be added to the holding tank. The last-resort method is to dump or inject chemicals directly into the holding tank. Once the additions are complete, it may take several hours for the suspension in the holding tank to respond to the additions.
When preparing daily batches, as much mixing and tuning should be performed in the main blunger as possible. Do not rush to transfer the new batch from the blunger into a holding tank if the suspension is not well-tuned. If production scheduling requires quick turnover, tuning can be a problem.
In one plant, batches were prepared in the main blunger. Some tuning was also performed in the main blunger. But suspension was transferred to a holding tank relatively quickly. The remainder of the tuning took the rest of the shift (i.e., the rest of the afternoon) to complete (and the word complete is used here with caution.) When procedures were changed to do as much tuning as possible in the main blunger, the batches were prepared, tuned, and completed before lunch.
Summary
Tuning suspensions in their holding tanks is a common procedure in many production plants. It simply must be done. But one must realize that the impellors in holding tanks (if well-designed) are there to provide recirculation from bottom to top to bottom, etc. Such impellors are not efficient when called upon to disperse additives and quickly produce homogeneity. If the impellors used are the water treatment type, suspension properties within the tank may vary with depth and distribution of tuning chemicals. Achieving homogeneity of distribution of additives in holding tanks is difficult. When water treatment impellors are used in holding tanks, it will be difficult just to obtain a representative sample of suspension. When such impellors are used, suspension properties frequently vary with depth -- which means any sample pulled from such a tank can vary as a function of depth from which is was pulled.
Holding tank impellors were not designed for efficiency of blunging -- so don't expect them to be efficient for such tasks. Don't expect them to be able to quickly and efficiently disperse a 'glug' of chemicals (added at the surface) throughout the whole holding tank volume. Don't even assume that the bottom of the tank is where it should be. Tank depths and capacities can vary due to hard-packed sediments that have built up with time and use. Process engineers MUST pay attention to the conditions in the bottoms of tanks, make sure that tanks are cleaned after each use, and make sure that impellors are intact and functional.
Whenever possible, do as much mixing, homogenization, and tuning in tanks designed for dispersion (i.e., in the main blungers.) Don't save any major dispersion requirements for storage tanks. Take care when tuning chemicals are added to any tank. So-called 'glugs' of chemicals are difficult to disperse -- even in blungers. Take special care when adding any types of chemicals to holding tanks. Extra long times will be required to achieve homogenization (if homogenization is indeed possible) in holding tanks.
Miscellany
'Glugs' .... I feel obligated to explain the definition of this word for those of you who use English as a second or third language. When chemical is poured from a bottle, it makes the sound, 'glug, glug, glug, ....' So a 'glug' represents a small volume (several cubic centimeters) of pure chemical.
Suggested topics .... Please continue to send your ideas or questions for future topics. Thanks. Until next time ...
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